Optical, infrared and millimetre-wave properties of Vega-like systems - III. Models with thermally spiking grains
Optical, infrared and millimetre-wave properties of Vega-like systems - III. Models with thermally spiking grains
复制标题
类 Vega 系统的光学、红外和毫米波特性 - III。
DOI：
--
复制
发表时间：
1997
期刊：
影响因子：
--
通讯作者：
M. Barlow
中科院分区：
文献类型：
--
作者：
R. J. Sylvester;C. Skinner;M. Barlow
类织女星是主序星，由于星周尘埃颗粒可能分布在圆盘中而表现出过量的红外辐射。我们最近发表了一个大样本的候选织女星样系统，包括光学，近红外和毫米波光度，和中红外光谱的观测数据库。在以前的论文中，我们提出了八个源的辐射传输模型，从我们的样本，有低的分数过剩光度。在这里，我们提出了进一步的八个来源的模型，所有的大部分过剩的发光度占主导地位的过量发射在近红外波长。结果发现，没有一个单一的尘埃颗粒分布在一个磁盘中的热平衡可以同时匹配的过量发射在近红外和更长的波长。我们试图将近红外发射建模为由于热尖峰小晶粒，其可以暂时达到产生过量近红外发射所需的高温。SAO 186777的近红外光谱显示了3.3-J. LmVIR发射带，证实了我们早期在较长波长处检测到的UIR发射，并表明小的碳质颗粒是造成一些近红外发射的原因。热尖峰模型只是部分成功，许多源需要在~ 1000 - 1500 K的热平衡下发射的晶粒的存在。这些颗粒必须要么非常靠近恒星（Au），要么由吸积光度提供动力。模型光盘的光学深度的计算表明，光盘在可见光波长的光学厚度，这些源的光学厚建模是可取的。圆盘在毫米波长下是光学薄的，使我们能够确认圆盘中存在大颗粒。本文中的恒星很可能比原型织女星更年轻。
Vega-like stars are main-sequence stars that exhibit excess IR emission due to circumstellar dust grains which are probably distributed in discs. We have recently published an obser vational data base for a large sample of candidate Vega-like systems, comprising optical, near IR and mmlsubmm-wave photometry, and mid-IR spectra. In a previous paper we presented radiative transfer models of eight sources from our sample that had low fractional excess luminosities. Here we present models of a further eight sources, all with large fractional excess luminosities dominated by excess emission at near-IR wavelengths. It was found that no single distribution of dust grains at thermal equilibrium in a disc could simultaneously match the excess emission at near-IR and longer wavelengths. We attempted to model the near-IR emission as due to thermally spiking small grains, which can temporarily attain the high temperatures required to produce excess near-IR emission. A near-IR spectrum of SAO 186777 shows the 3.3-J.Lm VIR emission band, confirming our earlier detection of UIR emission at longer wavelengths, and suggesting that small carbonaceous particles are responsible for some of the near-IR emission. The thermally spiking models were only partially successful and many of the sources required the presence of grains emitting in thermal equilibrium at ~ 10001500 K. These grains must either be located very close to the stars «1 au), or else be powered by accretion luminosity. Calculations of the optical depths of the model discs suggest the discs are optically thick at visual wavelengths; optically thick modelling of these sources is desirable. The discs are optically thin at mm wavelengths, allowing us to confirm the presence of large grains in the discs. The stars presented in this paper may well be younger than the prototype Vega-like stars.